Grant River Recreation Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grant River Recreation Area clear sky chart from 2005-11-06 to 2024-01-14.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 8% 3 6% 5% 3 4 5% 5% 7% 38%	74764
	Night	14% 8% 4 6% 5% 3 4 4 5% 6% 40%	75012
Transparency	Day	46% 5% 12% 23% 12% 1	74464
	Night	45% 4 12% 23% 14% 2	74832
Seeing	Day	18% 9% 28% 26% 14% 5%	14711
	Night	18% 8% 30% 31% 12% 1	24704

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 10% 6% 10% 6% 4 5% 5% 5% 6% 31%	1517
	11th to 20th	Day	12% 17% 6% 11% 7% 2 5% 3 5% 6% 26%	1464
	21st to end	Day	13% 14% 6% 11% 8% 3 4 4 4 7% 25%	1746
February	1st to 10th	Day	11% 14% 6% 8% 5% 3 4 6% 5% 7% 31%	1620
	11th to 20th	Day	15% 10% 3 7% 6% 2 5% 5% 4 7% 36%	1632
	21st to end	Day	14% 12% 4 10% 6% 3 4 4 4 6% 34%	1480
March	1st to 10th	Day	16% 9% 5% 8% 6% 3 4 4 5% 6% 36%	1810
	11th to 20th	Day	13% 8% 4 6% 6% 2 4 5% 6% 7% 39%	1980
	21st to end	Day	20% 11% 4 6% 5% 4 4 5% 5% 6% 30%	2291
April	1st to 10th	Day	19% 9% 4 7% 6% 4 4 4 5% 6% 32%	2160
	11th to 20th	Day	16% 11% 3 6% 5% 4 5% 5% 5% 6% 34%	2134
	21st to end	Day	21% 8% 3 6% 6% 3 4 4 5% 6% 34%	2160
May	1st to 10th	Day	16% 7% 3 7% 6% 4 6% 6% 6% 8% 29%	2468
	11th to 20th	Day	18% 8% 3 7% 6% 3 5% 5% 5% 7% 34%	2479
	21st to end	Day	16% 6% 3 5% 6% 4 6% 5% 6% 8% 36%	2725
June	1st to 10th	Day	12% 4 2 4 5% 3 5% 4 6% 9% 46%	2520
	11th to 20th	Day	12% 5% 2 6% 5% 3 4 5% 5% 8% 45%	2520
	21st to end	Day	11% 6% 2 5% 7% 3 5% 6% 7% 9% 40%	2520
July	1st to 10th	Day	8% 4 2 5% 4 3 5% 5% 7% 10% 48%	2520
	11th to 20th	Day	7% 4 2 5% 5% 3 6% 6% 6% 9% 47%	2520
	21st to end	Day	7% 5% 2 4 5% 3 5% 5% 7% 10% 46%	2743
August	1st to 10th	Day	7% 5% 2 5% 6% 4 5% 7% 8% 10% 42%	2369
	11th to 20th	Day	11% 6% 3 5% 5% 3 4 4 6% 8% 46%	2230
	21st to end	Day	11% 5% 2 5% 4 3 4 5% 5% 8% 48%	2376
September	1st to 10th	Day	15% 5% 3 4 5% 3 4 4 4 7% 46%	2160
	11th to 20th	Day	13% 4 2 5% 4 3 4 4 5% 8% 49%	2154
	21st to end	Day	15% 5% 2 4 5% 4 4 4 5% 7% 45%	1936
October	1st to 10th	Day	15% 6% 3 5% 4 3 4 4 4 5% 47%	1800
	11th to 20th	Day	20% 9% 3 6% 4 2 4 4 5% 6% 38%	1800
	21st to end	Day	19% 9% 3 6% 4 2 3 4 4 6% 41%	1883
November	1st to 10th	Day	12% 6% 3 6% 6% 3 4 4 5% 6% 45%	1672
	11th to 20th	Day	15% 11% 4 8% 6% 2 4 4 5% 6% 33%	1634
	21st to end	Day	15% 12% 4 6% 5% 2 3 4 4 7% 39%	1520
December	1st to 10th	Day	13% 13% 5% 10% 6% 4 5% 4 4 6% 32%	1520
	11th to 20th	Day	14% 13% 5% 8% 6% 4 5% 5% 5% 7% 29%	1918
	21st to end	Day	20% 21% 9% 10% 6% 3 3 3 4 4 17%	2783
January	1st to 10th	Night	11% 11% 6% 8% 7% 4 5% 4 4 5% 36%	2648
	11th to 20th	Night	13% 14% 7% 11% 7% 4 4 3 5% 5% 27%	2569
	21st to end	Night	15% 13% 6% 10% 6% 3 5% 4 4 7% 27%	2606
February	1st to 10th	Night	11% 13% 5% 9% 8% 4 6% 4 4 5% 31%	2340
	11th to 20th	Night	12% 9% 4 8% 6% 4 4 4 5% 6% 40%	2213
	21st to end	Night	16% 10% 5% 8% 5% 2 4 3 4 5% 37%	1776
March	1st to 10th	Night	18% 10% 4 6% 4 3 3 4 4 6% 37%	2160
	11th to 20th	Night	14% 6% 4 6% 6% 3 4 4 5% 7% 43%	1985
	21st to end	Night	21% 10% 3 7% 5% 3 5% 4 5% 6% 30%	2064
April	1st to 10th	Night	15% 9% 3 6% 5% 3 4 5% 4 6% 39%	1800
	11th to 20th	Night	19% 8% 3 5% 5% 3 4 4 5% 6% 38%	1775
	21st to end	Night	17% 8% 3 7% 5% 3 5% 5% 5% 7% 36%	1800
May	1st to 10th	Night	17% 8% 3 5% 6% 3 4 5% 5% 8% 36%	1490
	11th to 20th	Night	17% 7% 3 5% 6% 3 5% 3 5% 7% 39%	1416
	21st to end	Night	13% 5% 3 4 5% 3 4 4 5% 9% 45%	1551
June	1st to 10th	Night	8% 6% 2 5% 6% 2 4 5% 6% 6% 49%	1440
	11th to 20th	Night	10% 5% 2 5% 5% 3 4 5% 7% 9% 46%	1440
	21st to end	Night	11% 4 2 5% 5% 3 5% 5% 6% 8% 48%	1440
July	1st to 10th	Night	6% 3 1 4 4 2 4 4 5% 10% 56%	1440
	11th to 20th	Night	7% 3 2 3 4 2 3 3 6% 9% 58%	1440
	21st to end	Night	7% 2 2 4 5% 3 5% 5% 5% 8% 55%	1575
August	1st to 10th	Night	5% 4 2 4 4 3 4 5% 6% 9% 52%	1566
	11th to 20th	Night	9% 4 2 4 4 3 3 4 4 9% 53%	1670
	21st to end	Night	10% 4 2 4 5% 2 4 3 5% 7% 55%	1980
September	1st to 10th	Night	11% 5% 1 4 4 3 3 4 4 7% 54%	1800
	11th to 20th	Night	12% 4 2 5% 4 3 4 4 4 7% 52%	1800
	21st to end	Night	11% 3 2 4 4 3 4 4 5% 6% 53%	2007
October	1st to 10th	Night	13% 5% 2 4 4 3 4 4 4 5% 52%	2160
	11th to 20th	Night	18% 6% 3 4 4 3 4 4 4 7% 43%	2160
	21st to end	Night	17% 8% 3 5% 5% 3 4 4 5% 6% 39%	2471
November	1st to 10th	Night	12% 7% 3 5% 4 3 3 4 5% 6% 49%	2406
	11th to 20th	Night	15% 10% 4 7% 6% 3 4 4 5% 6% 34%	2541
	21st to end	Night	17% 9% 4 6% 5% 2 3 5% 5% 6% 38%	2660
December	1st to 10th	Night	14% 11% 5% 9% 7% 3 5% 5% 5% 6% 30%	2660
	11th to 20th	Night	15% 12% 5% 8% 6% 4 5% 4 4 6% 30%	3279
	21st to end	Night	21% 16% 7% 11% 7% 3 5% 4 4 4 18%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	54% 4 13% 17% 11%	1518
	11th to 20th	Day	60% 4 13% 17% 6% 1	1464
	21st to end	Day	62% 4 12% 14% 8% 1	1746
February	1st to 10th	Day	54% 3 14% 20% 8% 1	1620
	11th to 20th	Day	48% 5% 11% 23% 10% 2	1641
	21st to end	Day	53% 4 10% 20% 10% 2	1480
March	1st to 10th	Day	50% 4 15% 21% 9% 1	1810
	11th to 20th	Day	45% 5% 12% 24% 12% 2	1980
	21st to end	Day	56% 5% 10% 21% 7% 1	2291
April	1st to 10th	Day	53% 5% 9% 22% 11%	2160
	11th to 20th	Day	52% 5% 9% 21% 12% 2	2134
	21st to end	Day	52% 5% 9% 21% 12% 2	2160
May	1st to 10th	Day	52% 6% 13% 21% 8%	2468
	11th to 20th	Day	49% 5% 13% 21% 11% 1	2483
	21st to end	Day	47% 5% 11% 20% 14% 2	2725
June	1st to 10th	Day	36% 6% 14% 27% 16% 1	2520
	11th to 20th	Day	38% 6% 13% 23% 19% 1	2520
	21st to end	Day	40% 6% 13% 26% 13% 1	2520
July	1st to 10th	Day	32% 7% 14% 31% 13% 2	2451
	11th to 20th	Day	33% 8% 15% 27% 15% 2	2445
	21st to end	Day	33% 7% 15% 27% 15% 3	2591
August	1st to 10th	Day	35% 8% 15% 30% 11% 1	2354
	11th to 20th	Day	38% 5% 12% 28% 15% 2	2230
	21st to end	Day	34% 5% 12% 28% 18% 3	2376
September	1st to 10th	Day	39% 4 11% 25% 19% 2	2160
	11th to 20th	Day	34% 5% 11% 33% 16% 2	2154
	21st to end	Day	39% 5% 10% 29% 16% 1	1936
October	1st to 10th	Day	40% 5% 11% 29% 15% 1	1800
	11th to 20th	Day	48% 4 11% 23% 14% 1	1800
	21st to end	Day	47% 4 12% 22% 13% 1	1883
November	1st to 10th	Day	42% 3 13% 26% 14% 3	1672
	11th to 20th	Day	52% 3 13% 21% 10% 1	1634
	21st to end	Day	48% 4 17% 23% 8% 1	1520
December	1st to 10th	Day	55% 4 12% 19% 8% 2	1520
	11th to 20th	Day	54% 4 16% 15% 9% 1	1915
	21st to end	Day	72% 4 9% 11% 5%	2783
January	1st to 10th	Night	52% 4 11% 20% 12% 1	2648
	11th to 20th	Night	60% 4 14% 14% 7%	2569
	21st to end	Night	60% 4 11% 16% 9% 1	2606
February	1st to 10th	Night	56% 3 11% 19% 11% 1	2340
	11th to 20th	Night	47% 4 12% 23% 13% 2	2214
	21st to end	Night	50% 3 11% 22% 11% 3	1776
March	1st to 10th	Night	50% 4 12% 23% 10% 1	2160
	11th to 20th	Night	43% 3 13% 23% 16% 2	1985
	21st to end	Night	56% 4 9% 20% 9% 2	2064
April	1st to 10th	Night	47% 5% 10% 23% 14% 2	1800
	11th to 20th	Night	47% 4 8% 20% 16% 4	1775
	21st to end	Night	47% 5% 11% 20% 14% 3	1800
May	1st to 10th	Night	47% 5% 12% 23% 12% 1	1490
	11th to 20th	Night	46% 4 12% 23% 13% 1	1417
	21st to end	Night	38% 5% 13% 24% 18% 2	1552
June	1st to 10th	Night	34% 7% 13% 26% 18% 2	1440
	11th to 20th	Night	35% 5% 11% 24% 22% 3	1440
	21st to end	Night	34% 6% 16% 25% 16% 3	1440
July	1st to 10th	Night	25% 6% 14% 33% 18% 3	1400
	11th to 20th	Night	25% 6% 13% 32% 20% 4	1392
	21st to end	Night	26% 7% 17% 29% 18% 4	1487
August	1st to 10th	Night	27% 6% 15% 34% 15% 2	1557
	11th to 20th	Night	30% 5% 14% 30% 19% 2	1670
	21st to end	Night	30% 6% 11% 29% 19% 6%	1980
September	1st to 10th	Night	31% 5% 11% 27% 25% 2	1800
	11th to 20th	Night	33% 3 10% 33% 18% 3	1800
	21st to end	Night	32% 5% 9% 31% 21% 2	2007
October	1st to 10th	Night	34% 5% 10% 27% 22% 2	2160
	11th to 20th	Night	41% 4 11% 23% 19% 1	2160
	21st to end	Night	46% 4 11% 24% 14% 1	2471
November	1st to 10th	Night	37% 3 12% 26% 19% 2	2406
	11th to 20th	Night	51% 4 12% 21% 11% 1	2541
	21st to end	Night	48% 4 13% 26% 9% 1	2660
December	1st to 10th	Night	56% 4 12% 20% 8% 1	2660
	11th to 20th	Night	54% 5% 14% 19% 7% 1	3281
	21st to end	Night	69% 5% 8% 13% 5%	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	18% 14% 30% 26% 11% 1	270
	11th to 20th	Day	26% 11% 40% 16% 6% 1	249
	21st to end	Day	23% 13% 38% 18% 6% 2	264
February	1st to 10th	Day	18% 23% 39% 15% 3 1	240
	11th to 20th	Day	23% 22% 32% 16% 6% 1	240
	21st to end	Day	25% 21% 40% 11% 2 1	198
March	1st to 10th	Day	19% 20% 41% 16% 5%	240
	11th to 20th	Day	18% 20% 35% 20% 5% 3	240
	21st to end	Day	31% 10% 33% 18% 6% 1	377
April	1st to 10th	Day	28% 10% 33% 23% 6%	420
	11th to 20th	Day	25% 17% 26% 20% 11% 1	412
	21st to end	Day	25% 12% 30% 20% 11% 2	420
May	1st to 10th	Day	20% 10% 33% 26% 10% 1	569
	11th to 20th	Day	24% 9% 27% 25% 11% 3	625
	21st to end	Day	18% 8% 25% 26% 16% 6%	679
June	1st to 10th	Day	12% 4 26% 33% 20% 6%	630
	11th to 20th	Day	14% 5% 28% 33% 15% 4	630
	21st to end	Day	13% 5% 29% 32% 16% 6%	630
July	1st to 10th	Day	9% 3 15% 35% 25% 14%	621
	11th to 20th	Day	9% 4 27% 31% 23% 7%	618
	21st to end	Day	8% 3 25% 36% 22% 6%	684
August	1st to 10th	Day	8% 1 20% 34% 26% 11%	626
	11th to 20th	Day	14% 3 19% 29% 21% 15%	544
	21st to end	Day	13% 5% 18% 33% 22% 9%	495
September	1st to 10th	Day	17% 4 22% 30% 20% 6%	450
	11th to 20th	Day	13% 2 20% 35% 22% 8%	450
	21st to end	Day	17% 10% 25% 27% 17% 4	406
October	1st to 10th	Day	21% 6% 27% 30% 14% 2	270
	11th to 20th	Day	30% 11% 35% 16% 8%	270
	21st to end	Day	27% 12% 30% 23% 7% 1	297
November	1st to 10th	Day	17% 11% 39% 21% 10% 2	270
	11th to 20th	Day	20% 16% 33% 21% 8% 1	270
	21st to end	Day	25% 10% 35% 25% 5%	270
December	1st to 10th	Day	20% 13% 39% 21% 6% 2	270
	11th to 20th	Day	20% 10% 38% 22% 9% 1	270
	21st to end	Day	30% 12% 38% 16% 4	297
January	1st to 10th	Night	17% 14% 34% 26% 8% 1	950
	11th to 20th	Night	19% 11% 35% 28% 6%	918
	21st to end	Night	21% 10% 35% 29% 6% 1	990
February	1st to 10th	Night	18% 12% 39% 25% 6%	900
	11th to 20th	Night	18% 12% 37% 27% 5%	819
	21st to end	Night	25% 13% 35% 23% 4 1	592
March	1st to 10th	Night	23% 11% 35% 24% 6%	720
	11th to 20th	Night	18% 10% 33% 31% 8%	545
	21st to end	Night	25% 10% 33% 25% 6% 1	594
April	1st to 10th	Night	20% 12% 38% 25% 5%	540
	11th to 20th	Night	24% 10% 30% 29% 6% 1	533
	21st to end	Night	21% 7% 32% 30% 9% 1	540
May	1st to 10th	Night	20% 7% 32% 32% 9% 1	540
	11th to 20th	Night	19% 5% 30% 33% 12%	531
	21st to end	Night	16% 4 26% 35% 17% 2	584
June	1st to 10th	Night	12% 2 23% 47% 15% 1	540
	11th to 20th	Night	13% 3 26% 43% 14% 1	540
	21st to end	Night	12% 3 26% 43% 15% 1	540
July	1st to 10th	Night	7% 1 12% 43% 33% 4	525
	11th to 20th	Night	8% 2 19% 44% 25% 2	522
	21st to end	Night	8% 1 16% 47% 26% 2	591
August	1st to 10th	Night	7% 2 16% 44% 29% 2	537
	11th to 20th	Night	10% 1 11% 40% 31% 5%	534
	21st to end	Night	11% 2 14% 46% 24% 3	594
September	1st to 10th	Night	13% 2 17% 41% 21% 5%	540
	11th to 20th	Night	14% 1 19% 36% 26% 4	540
	21st to end	Night	13% 7% 27% 33% 19% 1	707
October	1st to 10th	Night	16% 5% 27% 31% 19% 2	720
	11th to 20th	Night	22% 9% 30% 30% 9%	720
	21st to end	Night	22% 10% 33% 27% 7% 1	792
November	1st to 10th	Night	16% 9% 38% 27% 9% 1	740
	11th to 20th	Night	21% 13% 37% 23% 6%	831
	21st to end	Night	21% 11% 34% 25% 8%	950
December	1st to 10th	Night	20% 11% 39% 23% 5% 1	950
	11th to 20th	Night	24% 9% 35% 27% 6%	950
	21st to end	Night	29% 11% 34% 23% 3	1045

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